The invention relates to digital-to-analog converters and, more particularly, digital-to-analog converters incorporating temperature compensation circuitry.
A pulse width modulation (PWM)-based digital-to-analog converter (DAC) converts digital input values into an analog output waveform. A typical PWM DAC generates an output pulse having a pulse width that is a function of a digital input value. A low pass filter circuit converts the PWM output pulse into an analog output waveform. A PWM offers a relatively inexpensive DAC implementation. Unfortunately, the low pass filter circuit has a high output impedance. For this reason, the PWM DAC ordinarily includes a buffer circuit for current amplification to amplify the analog output waveform produced by the low pass filter circuit. Some types of buffer circuitry include an undesirable temperature dependence, which is transferred to the analog output waveform. Buffer circuitry providing greater stability can be more expensive. Accordingly, the selection of a buffer circuit for a PWM DAC often requires a tradeoff between cost and accuracy.
In general, the invention is directed to a PWM DAC providing a low impedance output, thermal stability, and very low cost. The PWM DAC is especially useful in circuits that do not require absolute accuracy without calibration, but where medium resolution and temperature stability are desired. The PWM DAC includes an emitter follower buffer providing a low impedance current sinking output circuit that enables inexpensive implementation of a buffered PWM DAC. A bipolar transistor used in conventional emitter follower buffer circuits usually is susceptible to substantial temperature-induced variations in base-emitter junction voltage.
A PWM DAC, in accordance with the invention, provides a compensation circuit that adjusts the digital input value applied to the PWM to compensate for temperature-induced variation introduced by the emitter follower buffer circuit. In this manner, the invention is capable of providing a DAC implementation that is both stable with temperature changes and inexpensive.
The compensation circuit may incorporate circuitry that replicates the PWM DAC at least in part, and monitors the base-emitter voltage of a replicated emitter follower buffer to track temperature-induced variation in the PWM DAC. In some embodiments, the compensation circuit generates an error value that can be used to temperature compensate multiple PWM DACs within a particular system.
In one embodiment, the invention provides a DAC comprising a PWM that generates an output pulse having a pulse width as a function of a digital input value. A filter converts the output pulse into an analog output signal, and an emitter follower buffer circuit that buffers the analog output signal. In addition, a compensation circuit adjusts the digital input value to compensate for variation in the emitter follower buffer circuit.
In another embodiment, the invention provides a system comprising a first PWM DAC having a first emitter follower buffer circuit, and a second PWM DAC having a second emitter follower buffer circuit. A compensation circuit adjusts digital input values applied to the first and second PWM DACS to compensate for variation in the first and second emitter follower buffer circuits.
In an added embodiment, the invention provides a method for temperature compensating a DAC having a PWM that generates an output pulse having a pulse width as a function of a digital input value, a filter that converts the output pulse into an analog output signal, and an emitter follower buffer circuit that buffers the analog output signal. The method involves adjusting the digital input value to compensate for variation in the emitter follower buffer circuit.
In a further embodiment, the invention provides a DAC comprising a PWM that generates an output pulse having a pulse width as a function of a digital input value, means for converting the output pulse into an analog output signal, means for buffering the analog output signal, and means for adjusting the digital input value to compensate for variation in the buffering means.
The invention may provide one or more advantages. For example, the invention provides a PWM DAC that can be implemented using inexpensive emitter follower buffer circuitry without suffering from the temperature-dependence ordinarily associated with such circuitry. Instead, an inexpensive compensation circuit ensures that the PWM DAC produces an analog output waveform that is both stable and accurate over a wide range of operating temperatures. In addition, the compensation circuitry can be readily implemented in low-cost integrated circuitry, e.g., ASIC or FPGA.